S tructure determination of organic compounds Part 2

1. Structure determination of organic compounds Part 2

2. Nuclear Magnetic Resonance (NMR) Mass spectroscopy - size of molecule (molecular weight and formula) IR spectroscopy – functional groups Nuclear magnetic resonance NMRMap of carbon hydrogen framework

3. Earth’s magnetic field has a field strength of 2x10-5 tesla Magnet used in an NMR machine has a field strength of between 2-10 tesla Atomic nuclei act like tiny compass needles and have different energy when placed in magnetic field. The compass needle can rotate through 360º so have infinite number of different energy levels.

4. In absence of any external field the spins of magnetic nuclei are oriented randomly. When a sample containing these nuclei is placed between the poles of strong magnet, nuclei adopt specific orientation so that their tiny magnetic field is aligned parallel or antiparallel to external field. If we irradiate such oriented nuclei energy absorption will occur and lower energy state will flip to higher energy. When this flip occurs we say that nuclei is in a resonance with the applied radiation So the name nuclear magnetic resonance

5. Three types of nuclei Carbon 12C; Oxygen 16O(Even atomic number, even mass number) Not spinning, not possible to detect via NMR Odd atomic number or odd atomic mass the nucleus is spinning Spherical shape nuclei – spin number ½ - 1H; 13C; 15N; 19F; 29Si; 31P Quadrupolar nuclei – spin number I > ½ - 2H; 11B; 14N; 17O; 33S; 35Cl

6. Dissolve in a suitable solvent Put in a very strong magnetic field Irradiate with a short pulse of radiofrequency energy

7. Schematic operation of an NMR spectrometer The exact frequency required for nuclei resonance depends on: The molecule (identity of the nuclei) Strength of the external magnetic field

8. The difference in energy between the two spin states is dependent on the external magnetic field strength, and is always very small. The following diagram illustrates that the two spin states have the same energy when the external field is zero, but diverge as the field increases. At a field equal to Bx a formula for the energy difference is given (remember I = 1/2 and μ is the magnetic moment of the nucleus in the field). Stronger magnetic field – energy difference larger – higher frequency needed 1.4T – 60MHz - 1H-NMR 15MHz- 13C-NMR 1.41-4.7 T At high fields energy needed to flip is still only 0.1cal/mol so the method is the mildest among all spectroscopic energy given to measure IR is 10000 times greater

9. NMR – is using a radiofrquency radiation For spin 1/2 nuclei the energy difference between the two spin states at a given magnetic field strength will be proportional to their magnetic moments. For the four common nuclei, the magnetic moments are: 1H μ = 2.7927,19F μ = 2.6273, 31P μ = 1.1305 and13C μ = 0.7022. The following diagram gives the approximate frequencies that correspond to the spin state energy separations for each of these nuclei in an external magnetic field of 2.35 T. The formula in the colored box shows the direct correlation of frequency (energy difference) with magnetic moment (h = Planck's constant).

10. As an example, consider a sample of water in a 2.3487 T external magnetic field, irradiated by 100 MHz radiation. If the magnetic field is smoothly increased to 2.3488 T, the hydrogen nuclei of the water molecules will at some point absorb rf energy and a resonance signal will appear

11. When external field is applied the moving electrons set up tiny local magnetic fields so effective magnetic field is changed by local fields Shielding effect may be different for each nuclei So we can distinct NMR signal for each chemically different hydrogenor carbonnuclei

12. 1H-NMR Methyl acetate intensity Chemical shift 13C-NMR and 1H-NMR cannot be observed at one time – they require different amounts of energy to „flip” spins of nuclei 13C-NMR intensity Chemical shift

13. General features of 1H-NMR spectra • Chemical shifts • Number of NMR absorptions • Integration of NMR absorptions • Spin – spin splitting pattern

14. Chemical shift Increasing field Lowfield Downfield Nuclei deshielded Upfield Highfield Nuclei shielded TMS signal By convention set to 0 Chemical Shift (d) = frequency observed (Hz)/oscillator frequency (MHz) = ppm 8ppm 7ppm 6ppm 5ppm 4ppm 3ppm 2ppm 1ppm 0ppm 1ppm = 60Hz for apparatus working at 60MHz 1ppm = 250Hz for apparatus working at 250MHz

15. Chemical shifts ! (important) Most proton chemical shifts range 0-10ppm If the electron density about a proton nucleus is relatively high, the induced field due to electron motions will be stronger than if the electron density is relatively low. The shielding effect in such high electron density cases will therefore be larger, and a higher external field (Bo) will be needed for the rf energy to excite the nuclear spin. Since silicon is less electronegative than carbon, the electron density about the methyl hydrogens in TetraMethylSilane is expected to be greater than the electron density about the methyl hydrogens in neopentane (2,2-dimethylpropane), and the characteristic resonance signal from the silane derivative does indeed lie at a higher magnetic field. Such nuclei are said to be shielded. Elements that are more electronegative than carbon should exert an opposite effect (reduce the electron density); and, as the data in the following tables show, methyl groups bonded to such elements display lower field signals (they are deshielded). The deshielding effect of electron withdrawing groups is roughly proportional to their electronegativity.

16. Chemical shifts for different protons

17. Proton equivalence Chemically distinct hydrogen has its own unique absorption – so we need to find how many different hydrogens are present in a structure RULE: If the protons are chemically equivalent, the same product will be formed regardless on which proton is replaced. If protons are not chemically equivalent, different products will be formed on substitution. δ = 1.71 ppm 2,3-dimethylbut-2-ene

18. Dipropyl ether

19. Proton Chemical Shift Ranges Low field region High field region

20. Regions of the 1H-NMR spectrum

21. CH4 methane δ = 0.23 ppm CH3Cl chloromethane δ = 3.06 ppm CH2Cl2 methylene chloride δ = 5.47 ppm CHCl3 chloroform δ = 7.26 ppm

22. Integration of 1H-NMR – proton counting The area ofsignal is proportional to the number of protons creating that signal. By integrating this area it is possible to measure the relative number of each kind of proton in molecule The magnitude or intensity of NMR resonance signals is displayed along the vertical axis of a spectrum, and is proportional to the molar concentration of the sample. Thus, a small or dilute sample will give a weak signal, and doubling or tripling the sample concentration increases the signal strength proportionally. If we take the NMR spectrum of equal molar amounts of benzene and cyclohexane in CDCl3 solution, the resonance signal from cyclohexane will be twice as intense as that from benzene because cyclohexane has twice as many hydrogens per molecule. This is an important relationship when compounds with twoor more different sets of hydrogen atoms are examined, since it allows the ratio of hydrogen atoms in each distinct set to be determined

23. Diethyl ether 1.5 1:1.5 4:6 1

24. 2-chloro-1-ethoxy-5-hexen-3-one Integration 1:2:1:1:1:2:2:3

25. Spin-Spin Interactions The splitting patterns found in various spectra are easily recognized, provided the chemical shifts of the different sets of hydrogen that generate the signals differ by two or more ppm. The patterns are symmetrically distributed on both sides of the proton chemical shift, and the central lines are always stronger than the outer lines. The most commonly observed patterns have been given descriptive names, such as doublet (two equal intensity signals), triplet (three signals with an intensity ratio of 1:2:1) and quartet (a set of four signals with intensities of 1:3:3:1). The line separation is always constant within a given multiplet, and is called the coupling constant (J). The magnitude of J, usually given in units of Hz, is magnetic field independent.

26. What causes this signal splitting, and what useful information can be obtained from it ? If an atom under examination is perturbed or influenced by a nearby nuclear spin (or set of spins), the observed nucleus responds to such influences, and its response is manifested in its resonance signal. This spin-coupling is transmitted through the connecting bonds, and it functions in both directions. Thus, when the perturbing nucleus becomes the observed nucleus, it also exhibits signal splitting with the same J. For spin-coupling to be observed, the sets of interacting nuclei must be bonded in relatively close proximity (e.g. vicinal and geminal locations), or be oriented in certain optimal and rigid configurations. Using this terminology, a vicinal coupling constant is 3J and a geminal constant is 2J.

27. Protons chemically equivalent do not exhibit spin-spin splitting No splitting occurs Protons having n equivalent neighboring proton gives signal splitted into n+1 lines Splitting observed Splitting not observed

28. Splitting of 1H-NMR signals Ho Hproton

29. Splitting of 1H-NMR signals

30. Splitting of 1H-NMR signals If a given nucleus is spin-coupled to two or more sets of neighboring nuclei by different J values, the n+1 rule does not predict the entire splitting pattern. Instead, the splitting due to one J set is added to that expected from the other J sets. Bear in mind that there may be fortuitous coincidence of some lines if a smaller J is a factor of a larger J.

31. Summary of1H-NMR spectra • Chemical shifts – kind of hydrocarbon framework • Number of NMR absorptions – groups of chemically equivalent protons • Integration of NMR absorptions – relative number of different protons • Spin – spin splitting pattern – groups of neighbouring protons

32. Carbon framework – 13C-NMR 1.1% of elemental carbon is the 13C isotope, which has a spin I = 1/2, so it is possible to conduct a carbon NMR experiment 1.  The abundance of 13C in a sample is very low (1.1%), so higher sample concentrations are needed.2.   The 13C nucleus is over fifty times less sensitive than a proton in the NMR experiment.3.   Hydrogen atoms bonded to a 13C atom split its NMR signal by 130 to 270 Hz, further complicating the NMR spectrum

33. 1H-NMR spectrum of camphor.

34. 13C-NMR spectrum of camphor.

35. 13C-NMR spectrum of camphor Decoupling of carbon-hydrogen splitting When spectrum acquired in this manner, the carbon NMR spectrum of a compound displays a single sharp signal for each structurally distinct carbon atom in a molecule.

36. Part of 13C-NMR spectrum of camphor

37. Identifying the molecule by 13CNMR Low field region High field region

38. UV-VISSPECTROSCOPY

39. UV-vis spectroscopy uses ultraviolet and visible light

40. UV-vis spectroscopy (200 - 800 nm) Violet:   400 - 420 nm Indigo:   420 - 440 nm Blue:   440 - 490 nm Green:   490 - 570 nm Yellow:   570 - 585 nm Orange:   585 - 620 nm Red:   620 - 780 nm

41. UV-vis spectroscopy Most spectrometers display absorbance on the vertical axis, and the commonly observed range is from 0 (100% transmittance) to 2 (1% transmittance). The wavelength of maximum absorbance is a characteristic value, designated as λmax. Absorption may be presented as: transmittance (T = I/I0) or absorbance (A= log I0/I).

42. UV-vis spectroscopy Absorbance of a sample is proportional to the number of absorbing molecules in the spectrometer light beam (e.g. their molar concentration in the sample tube), it is necessary to correct the absorbance value for this and other operational factors if the spectra of different compounds are to be compared in a meaningful way. The corrected absorption value is called "molar absorptivity, (or extinction), and is useful when comparing the spectra of different compounds and determining the relative strength of light absorbing functions (chromophores). Molar absorptivity (ε) is defined as:

43. Electronic transitions in unsaturated molecules

44. UV-vis spectroscopy

45. UV-vis spectroscopy Molecular moieties likely to absorb light in the 200 to 800 nm region are π-electron functions and hetero atoms having non-bonding valence-shell electron pairs. Such light absorbing groups are referred to as chromophores orange β-carotene

46. Empirical Rules for Absorption Wavelengths of Conjugated Systems Woodward-Fieser Rules for Calculating the λmax of Conjugated Dienes and Polyenes

47. Woodward-Fieser Rules for Calculating the π __>  π* λmax of Conjugated Carbonyl Compounds λmax (calculated) = Base + Substituent Contributions and Corrections